National Center for Atmospheric Research
ATD... SHEBA ISFF Flux-PAM Project Report

5.0 Barometers

Atmospheric pressure was measured with Vaisala PTB 220B digital barometers located inside the large insulated boxes containing
pressure port
J. Militzer
the thermoelectric generators and the station data system. The total accuracy of these sensors, including non-linearity, hysteresis, drift, calibration uncertainty etc., is specified by Vaisala to be ±0.3 mb. In order to minimize dynamic pressure errors, a Nishiyama-Bedard quad-disk pressure port (shown at left, coated with rime) was mounted on the upper surface or lid of the insulated box and connected to the barometer with a flexible hose. This port is expected to reduce dynamic pressure errors to less than 0.1 mb. No problems were encountered with the barometers during the SHEBA field program.

6.0 Hygrothermometers

Atmospheric temperature and relative humidity were measured with NCAR hygrothermometers (TRH) consisting of a Vaisala Humitter 50Y sensor interfaced to NCAR microprocessor electronics and enclosed in a mechanically-aspirated NCAR radiation shield. The Vaisala 50Y sensors employ a platinum-resistance temperature (PRT) transducer and an Intercap capacitive humidity transducer. Since the 50Y PRT has a lower limit of -40 °C, it was supplemented for the SHEBA project with a thermistor calibrated over the range -55 °C to 10 °C. The temperature and humidity sensors were calibrated prior to the SHEBA field project in the NCAR Sensor Calibration Laboratory and the derived calibration coefficients were entered into EPROM memory in the individual TRH electronics. The NCAR hygrothermometer outputs a serial message containing fully-calibrated temperature(s) and relative humidity (with respect to water) data.

The TRH electronics were also used to ingest data from the electronic magnetometer compass. Since the compass data are necessary for determination of wind direction, they are discussed in the section of the report devoted to the sonic anemometers.

Temperature and humidity calibrations were performed at NCAR both before and after the SHEBA field project. The post-project temperature calibrations were nominally within 0.1 °C of the pre-project calibrations for both the platinum resistance and thermistor transducers. During the post-project humidity calibrations, the outputs from four of the 50Y sensors were less than the pre-project calibrations by 2-4 %RH, while a fifth sensor (TRH006) was less by 4-8 %RH. The cause of the departure of TRH006 from the other NCAR 50Ys is not known. Additional post-project laboratory tests were performed to determine the performance of the 50Y sensors near the frost point at low temperatures. The results of those tests are found in Appendix A.

The sign of the humidity calibration shift is also somewhat surprising, because contamination of the capacitive transducers would be expected to increase rather than decrease the sensor output. However, a similar shift (relative to the manufacturer's original calibration) was found in the post-project calibration at NCAR of Vaisala HMP 35 sensors used on the ASFG 20-m tower. A smaller shift of only 1-2 %RH was found for an ASFG hand-held HMP 35 unit, which had much less exposure to the environment over the course of the project. The humidity sensors were calibrated at -15 °C in a Thunder Scientific 2-pressure humidity chamber, with a manufacturer-specified accuracy of ±1 %RH. More details on the calibration procedures and results are available on request from NCAR.

The hygrothermometer radiation shield is composed of two concentric cylinders made of thin-walled brass tubes,
E. Andreas, October 25, 1997
10.75 inches long and with diameters of 1.0 and 1.375 inches. The cylinders are oriented vertically, with the sensor located on their common axis and recessed 7.75 inches above the inlet. The exterior of the shield is painted white and the interior is aspirated with a Conair, 2 inch diameter, 12 VDC fan. After the start of the SHEBA project, we found that the aspiration rate of this shield decreases with increasing wind speed. Measurements are planned to quantify the resulting temperature measurement error as a function of wind speed and radiation load. This information will be included in the project report when it is available. (Following the SHEBA project, the inlet of the radiation shield was modified to alleviate the aspiration problem.) Another potential problem is that frost often covered the shield inlet. The effect of this on the measurements is unknown, but could be investigated by examining the data before and after cleaning of the inlet during station maintenance visits.

Table 6.1 lists the heights of the TRH radiation shield inlets measured relative to the local surface. (On August 24-26, the heights of the hygrothermometer booms were measured at stations 2-4, rather than the inlet heights, and the heights of the inlets were estimated by subtracting 43 cm from those measurements.) The heights of the sonic booms were measured more often than the heights of the hygrothermometer inlets and are also listed in Table 6.1 so that they can be used to estimate the inlet heights when the inlet heights were not measured directly. The variability of the difference of these two heights is perhaps caused by lack of spatial uniformity of the surface.

Table 6.1. Heights of TRH inlets and sonic booms
Station 1   Station 2
Date TRH Sonic   Date TRH Sonic
97 10 15  2.88m   97 10 15  2.88m
98 04 11   2.26m   98 04 11   2.82m
98 04 17   2.26m   98 04 16   2.87m
98 05 21 1.27m 2.22m   98 05 16 1.81m 2.87m
98 06 12 1.43m 2.39m   98 06 12 2.04m 3.02m
98 06 22 1.61m 2.51m   98 06 23 1.92m 3.03m
98 07 06   2.77m   98 07 07   3.12m
98 07 21   2.87m   98 07 23   3.24m
98 08 03   2.92m   98 07 30   3.23m
98 08 29 1.87m 2.97m   98 08 26 2.18m 3.28m
98 09 16 1.84m 2.90m   98 09 16 2.30m 3.26m
98 09 30 1.69m 2.73m        

Station 3   Station 4
Date TRH Sonic   Date TRH Sonic
97 10 15  2.87m   97 10 231.73m 2.90m
98 04 22   2.46m   98 04 11   2.75m
98 05 15 1.43m 2.34m   98 04 20   2.62m
98 06 18 1.71m 2.63m   98 05 16 1.88m 2.84m
98 06 29 1.80m 2.68m   98 06 08 1.87m 2.89m
98 07 09   2.69m   98 06 13 1.86m 2.90m
98 07 30   2.97m   98 07 29   2.93m
98 08 10   2.97m   98 08 26 1.87m 2.95m
98 08 24 1.94m 2.93m   98 09 08 1.87m 2.93m
98 09 20 1.90m 2.92m   98 09 20 1.96m 2.92m
        98 09 30 1.82m 2.94m

Andreas and Claffey have provided 1 hour estimates of TRH and sonic height above snow, based on the above measurements and snowline data from Don Perovich's group. These estimates are documented in an appendix to this report.

Six NCAR hygrothermometers were used during SHEBA field operations. Table 6.2 lists the history of the TRH serial numbers for each station. Each row represents a change in the hygrothermometer at one of the stations. Also included in the table are the apparent reasons for the changes and a reference to the relevant logbook entry. Only one or two changes appear to be associated with failures of the temperature and humidity sensors. Many of the TRH changes were associated with upgrades or repairs that were implemented by successive swaps among the stations. These included an improvement to the radiation shield mount, installation of sonic heaters, and repair of a beacon. (The sonic heaters and beacon were controlled by the TRH microprocessor.) The TRH aspiration fan occasionally attracted the attention of a passing polar bear and thus two changes were needed to repair bear damage. Note that TRH001 saw limited use before failing and did not receive a post-project calibration.

Table 6.2. TRH serial numbers at each station
Date GMT 1 2 3 4 Comment Logbook #
    0003 0004 0005 0006 installed  
97 10 28 18:50 0003 0004 0005 0007 improve mount 93
97 10 28 23:35 0003 0006 0005 0007 improve mount 93
97 10 29 19:20 0004 0006 0005 0007 improve mount 97
97 10 30 00:50 0004 0006 0003 0007 improve mount 98
97 12 24 21:55 0004   0006 0007 suspicious RH? 170
97 12 27 20:30 0004 0005 0006 0007 bear repair 125,155,173
97 12 31 23:35 0004 0005 0006 0003 test 180
98 01 08 06:10 0004 0005 0006 0007 sonic heaters 195
98 01 13 20:35 0004 0005 0007   sonic heaters 213
98 01 14 03:15 0004 0005 0007 0006 sonic heaters 212
98 01 15 19:55 0006 0005 0007   sonic heaters 217
98 01 21 00:40 0006 0005 0007 0003 sonic heaters 225
98 02 03 22:15   0005 0007 0006 repair beacon 243
98 02 09 00:40 0003   0007 0006 repair beacon 242
98 02 26 01:15 0003   0007 0005 test? 280
98 03 31 01:05 0003   0001 0005 bear repair 344
98 04 06 22:35 0003 0006 0001 0005 resurrected stn 2 362
98 05 11 22:45 0003 0006 0001 0007 check spare 461
98 05 25 22:20 0003 0006 0005 0007 bad RH 485

At the start of the project, there was spiking at a rate of roughly once per 2 hours on the data output by the NCAR hygrothermometer, including the data from the Vaisala 50Y, the supplementary thermistor, and the electronic compass. The cause was traced to a spurious character that intermittently occurred in the serial output message of the hygrothermometer, and this was fixed by modifying the EVE station software configuration to ignore the spurious character. The new configuration was downloaded at station 4 on October 23, at station 1 on October 24, and at stations 2 and 3 on October 26. These spikes were removed during post-project data processing by applying a 1-hour median filter to the hygrothermometer data during this period. Note that this filter also suppresses valid maxima or minima within the 1-hour window in the same manner as erroneous spikes, reducing the variance of the time series.

During SHEBA field operations, it was noted that the 50Y PRT was usually from 0 to 0.2 °C warmer than the thermistor, although the laboratory calibration of both transducers is accurate to better than 0.1 °C. The PRT is the standard Vaisala 50Y temperature sensor and is enclosed, along with the humidity sensor, in a porous shield designed to protect the humidity sensor from contamination. The thermistor was mounted external to the porous shield. We suspect that the PRT was not dissipating its self-heating as effectively as the thermistor sensor, because the porous shield restricts the heat exchange with the ambient air. This explanation is supported by a series of simple tests carried out in early March 1998: when the porous shield was removed, the temperature of the PRT was no longer higher than that of the thermistor, and when it was replaced, the PRT was again warmer than the thermistor. Thus the thermistor data might be expected to better represent the ambient air temperature, but the 50Y PRT data should be used (above -35 °C) to convert the 50Y relative humidity measurements to ambient mixing ratio. The hour-average air temperatures calculated during post-project processing of the data are the PRT temperature above -35 °C (as determined by the PRT) and the thermistor temperature below.

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